You maybe send them a PM or tag them in the OOC to check if they're coming back (illness, work, family, etc. may have kept them off site unexpectedly).
If there's no response, you shrug and move on, and maybe write them off for future games.
Also it varies on what constitutes inactivity. Look to see how active the rp was prior to the GM going missing. If a post a week is average you might have to wait awhile before they come back. If it's like multiple replies a day than as @Grey said I'd contact them to see what's up.