Rez-bots | Revive fallen units or buildings
Select a resurrection unit (Bot or Sub) and Right Click on any wreckage to revive that fallen unit.
Resurrection costs quite some energy, so make sure you have enough of that.
TIP: You can resurrect a lot faster (and more) with the Area Resurrect Command
PS. You can only resurrect wreckages that are still partly in tact. Rubble and debris cannot be resurrected.