Bitcoin-qt might minimize to system tray. Does it say "Cannot obtain lock on data directory ... ... Bitcoin is probably already running." when you attempt to open it again?
Well, in that case bitcoin is already running, like it says. All you have to do is open it from your system tray, which is located at bottom right of your screen should you be using a Windows OS of some sorts.
This had me wondering a couple of times. Especially if your PC runs 24/7 and you have Windows hide inactive icons in the tray, you can forget you had it running. Upside is you don't have to wait for it to recheck the blockchain