bathroom tiles are popping up

i have a few ceramic tiles in my bathroom that are popping up. the grout around the tiles are gone. do i have to remove the tile and redo, or can i just regrout it and seal the tiles.

 

If the tiles are loose they cant be simply regrouted, pull them up, clean off the old cement from the tiles and the floor. Then endeavor to find out why they came loose because chances are they will come loose again if the underlying problem is not corrected.
Some of the more frequent conditions leading to loose tile are:
1.) water got under and started to rot out the subfloor causing the tile bond to break.
2.) improper installation materials under the tile. Could be the type of cement that was used, vinyl and or luan under the tile, not enough strength in the subfloor to support the tiles.
3.) Tenting, caused by movement of the building pressing on tile or grout installed directly up against the walls of the room.
4.) improper installation techniques. The thinset could have skinned over before the tile were placed in position, tiles not pressed into thinset enough, not mixing the thinset properly prior to spreading it on the floor, somebody walked on the tiles before they set up breaking the bond.

There could be other things also but the above are the usual suspects. It might be best to call in a local pro and get an opinion from him/her. Come back here with your findings.