Not that big of a deal, people lose their ID cards from time to time. It happens and as long as you weren't doing something stupid with it it is relatively simple to replace.
1- Download the attachment on this post and fill it out as much as possible. This is what gets forwarded to the Pl comd/OC and eventually the CO for signature. Hand it in ASAP Monday or your next Pde night. The unit will do a quick preliminary investigation to make sure everything is kosher.
2- From there it will be forwarded to Base Ident sect. They will do their stuff and either call you in or have you make an appt (depends on the base and their policies).
3- When you go in you may have to talk to a MP about the loss of the ID card. Sometimes I've seen it sometime not. It really depends, but all in all it isn't that big of a deal.