I found an old wallet.dat, but I cannot import it and I don't know why.
First off, is this your wallet. I know this is a dumb question but I always have to ask. Also, was this wallet "encrypted"? What version of the wallet did you use and was this wallet based on the same application you are using now.
For example, Bitcoin Wallet Qt, you have to match it with the same product. I've noticed for example if I tried my wallet import doesn't work on other Bitcoin wallet applications, even if I have the password for the encryption correct.
This should really be answered by the developer of said wallet and since I don't know which wallet you're using, I can't help you at the current time in this regard.