Please take the time to read and understand this complete post before following any instructions.
If you have backups of your wallet.dat file to several locations (such as copying it to removable media plus another location on your hard drive), than your bitcoins will not be lost. The issue will be getting the Bitcoin client working and caught up so that you can actually
spend your money.
Do you have at least 5GB of hard drive space free?
At this point, I would be looking at possible problems on your computer, such as bad sectors on the disk or bad RAM. Bitcoin uses processor and disk-intensive processes when downloading blocks which may reveal computer problems. Even a problem like a bad CPU fan or a CPU heatsink clogged up with dust and lint could cause processing errors on a high-CPU-usage program like Bitcoin.
To avoid ambiguity, I'll tell you exactly what I would do in this situation (these instructions assume English Windows 7 or similar):
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A. Verify wallet is backed up properly:----
1. Restart computer
2. Verify Bitcoin is not running (Press CTRL-ALT-DEL key combination, start task manager, and view the applications and processes tabs to ensure you do not have bitcoin-qt.exe running).
3. To avoid mistakes, disable Windows' file name and extension-hiding: In the Windows "Control Panel" -> "Folder Options", select the "View" tab. Uncheck the boxes next to "Hide extensions for known file types", and also select "Show hidden files...". Press OK.
4. Open the %appdata%/Bitcoin data directory using Windows Explorer (note the actual directory name is similar to C:\Users\BrendStatter\AppData\Roaming\Bitcoin, using whatever your Windows user name is)
5. Copy the wallet.dat file to several locations, such as a USB stick, onto your Windows desktop, etc. so that it will never be lost (also keep these backups secure, if someone else got your wallet.dat file, they could spend your bitcoins). Verify the file is properly copied to the new locations.
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B. Computer checks---
There are several utilities I would recommend to ensure your computer is working well.
Check disk for errors:
-type "Command Prompt" in the start menu search. Right-click on the found "Command Prompt" program, and choose "Run as administrator".
-in the command prompt that opens, type
chkdsk /f c:-you will be prompted if you want to check next time the computer restarts, press Y (and enter) to schedule a disk check
-restart the computer
(advanced check, type
chkdsk /r c: instead, to additionally scan for bad disk sectors. This may take from 5 minutes on a small SSD to 10 hours on a slow 2GB drive, but will ensure the entire surface area of your disk is ok)
Memory check:
In the start menu, run "Windows Memory Diagnostic", and choose "restart now and check", let the computer restart to do a memory test.
More advanced: Download
memtest86+, and burn the ISO disk image to a CD, restart your computer booting off that CD, and let the memory test run for several hours until it has passed several times.
CPU test:
Try the burn-in program
OCCT. It has 100% CPU usage tests such as the default test and Linpack that will make your CPU hot, and will also report to you if any error has happened in calculation. This test must pass - if it fails at all, your CPU is getting too hot or something is bad.
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C. Remove Bitcoin data directory and retry block download---
Just like it says. After you are 110% sure you have multiple wallet backups, completely remove the Bitcoin data directory (we'll just rename it):
1. Ensure Bitcoin is closed
2. Type %appdata% into the Windows start menu and press enter to open the Windows application data folder
3. Locate the "Bitcoin" directory
4. Right-click on the "Bitcoin" directory, choose "Rename", and rename it to "Bitcoin-Bad", "Bitcoin-Original" or such.
5. Start Bitcoin, it will act like a brand new install, creating a new data directory, new wallet, and will begin downloading from block 1.
6. Do not interrupt Bitcoin - give it 6+ hours or more to download the complete blockchain until it is caught up (Note: after block 180,000, download starts becoming much slower)
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D. Restore your original wallet---
Hopefully all that worked, and you have an in-sync fresh copy of Bitcoin with 0 bitcoins. Now we need to put your original wallet into the working Bitcoin so you can spend your money!
1. Close Bitcoin and the Bitcoin tray icon, make absolutely sure it is closed (it may run up to a minute in the background after you close it)
2. Type %appdata% into the Windows start menu and press enter to open the Windows application data folder
3. In the "Bitcoin" directory, rename the wallet.dat file to something like "wallet-empty-good-whatever.dat"
4. Locate the "Bitcoin-Bad" backup directory or USB backup that contained your original wallet.dat
5. Copy your original wallet.dat file into the %appdata%/Bitcoin directory
6. Restart Bitcoin this way:
Paste the line
"\Program Files (x86)\Bitcoin\bitcoin-qt.exe" -rescan into the start menu search bar. (remove the " (x86)" if you are using 32 bit Windows).
7. It will take several minutes for Bitcoin to start up, because it will look through the new complete blockchain for any payments to you.
8. Send your money!