Should probably give a bit more info, I assume you're using the GUI?
If you don't open the wallet for a couple weeks it seems logical to me that it might show that it is out of sync, because it is.
Both Bitcoincore, wallet.dat and block chain are on a external driver
I assume you didn't change the paths inbetween now and when you succesfully used the wallet for a couple months?
A command-line approach (what I would do) would be to first check if your bitcoin core instance is synced. then load the wallet.dat and check the balance.
So start bitcoind from command line using whatever bitcoin.conf or startup options concerning your paths to your external drive,
bitcoin-cli -rpcuser=<user>-rpcpassword=<pass> getblockchaininfo
...
bitcoin-cli loadwallet "test.dat"
Hm. This is probably not ideal for a beginner.
What you should do is back up your wallet.dat to a secure location before you do anything. If nothing works out you can always load the wallet.dat in a fresh core download and export your private keys to your inputs manually to another wallet such as electrum.
Load your wallet into unsynced bitcoin core > console > start generating receive addresses / raw change addresses > export private keys int electrum?
I assume this process is deterministic and at some point you should've exhausted all used addresses? Someone should comment on this. There's probably an easier solution to your problem though, so maybe just ignore this post.