What happens when domain expires?
 

I had a domain... nothing special but it had some sentimental value... it expired the other day and I didn't realize it... is there anything I can do to get it back? If not how long before I can re register it?

Go re-register it. It goes into a redemption period and I believe you can get it back up intil it actually gets released for registration. Not 100% on that but since you own it the registrar should let you do that. If you do actually have to let it drop then register it you need to go to one of these domain buying sites such as pool.com or snap names

Depends on the policies of your registrar.. Supposed to fall into a grace period (and some companies charge top dollar for to reregister during that time). Once beyond the grace period, good domains often get sniped by partnered services for backorders (snapnames) or auctions (namewinner)