Lost mining shares when restarted GUIMiner
Your shares are towards a block, once a block is completed you start on a new block and go back to 0 shares. Depending on your pool, you get paid per block or per share. Don't worry, if you look on your pools website you should be able to find what you made off of the block and how many shares you contributed.

Restarting GUIMiner will not remove your shares, your shares are automatically recorded through your pool.