Make sure you have enough space and your anti virus soft (if any) is not messing with the raw blockchain data. There are some virussignatures stored in there, but thats not an actual thread.
If the database is not working, you have to rebuild it. Since this only needs CPU time you dont actually have to redownload anything. In order to let it rebuild you either accept the promt with "yes" or you run it with:
-reindex [1]
in order to sync faster you should use a fast node in your area and only connect to that node. See the list below.
run bitocin core with
-connect=IP where you replace the IP with one from the list.
last updated 2014.11.25
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IP - location - owner[1] - speed - info/stats page[2] - testnet
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eldinhadzic.org[4] - LA, USA - eldinhadz - 1000 mbit/s - no - no
84.200.34.113 - Freinsheim, DE, EU - Newar - 1000 mbit/s - yes - no
185.45.192.129 - Amsterdam, NL, EU - anon - 1000 mbit/s - /node.php[3] - yes
213.165.91.169 - Germany, EU - shorena - 100 mbit/s - yes - no
50.7.68.180 - New York, USA - Newar - 100 mbit/s - yes - no
5.9.24.81 - Germany, EU - zvs - unknown - no - no
178.79.173.71 - United Kingdom, EU - zvs - unknown - no - no
107.155.104.194 - Dalls, USA - zvs - unknown - no - no
106.185.32.195 - Japan, Asia - zvs - unknown - no - no
94.242.57.173 - Russia, Asia - zvs - unknown - no - no
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[1] refers to a bitcointalk.org username or anon if requested
[2] same IP, port 80 or path/port given
[3] work in progress
[4] IP is 173.236.158.177
Once you are dony snycing start bitcoin core like you usually would. While its possible it makes no sense to connect to more than one node as only the first is used for syncing.
Should you for whatever reasons have to redownload the entire blockchain, use the torrent [2].
[1]
https://en.bitcoin.it/wiki/Running_Bitcoin[2]
https://bitcointalk.org/index.php?topic=145386.0