Lost Litecoin wallet? Using Litecoin-QT

First I created wallet a. Then I sent 2 LTC to that wallet's public address.

Then I imported a private key in Litecoin-QT and wallet a was gone?! Aren't all .dat files  that has been used stored somewhere, or do they get overwritten everytime you import a new key to your wallet?
Importing a private key should not cause you to lose your wallet. If you import a private key and then restore from a previous backup, you may lose that key, however.