GUIminer is a UI front-end for Bitcoin mining software. You can use it to mine on pools or solo but does not hold any accounts whatsoever.
You will have to find the pool that you used to mine Bitcoin on if that particular pool stored your mined Bitcoin there, or find the wallet that you sent the mined Bitcoin to automatically.

If GUIminer stored settings on a file, then you can probably use that file to find the pool you used to mine on assuming it still exists.