First, check to make sure that your fuse is intact. This is the large blue fuse in the lower-left hand corner of the RAMBo card. Simply pull it out, and check to make sure it looks like this:
If the fuse is not intact, you can buy a standard 15-amp car fuse from any retailer to replace it with.
If the fuse is intact, check to make sure the wiring on the bottom of the heat bed are securely soldered to the bottom of the heat plate. You should be able to lift your printer to check this.
If the soldering has somehow come undone, you may try to solder the cables back to the heat plate.
If the soldering has not come undone, please contact us for further assistance.