My thermostat was loose so I removed it from the wall to put in a couple of anchors to make it tight and the red wire came off, the screen went blank, but then I changed the batteries and a number 3 came on but still nothing works

You didn`t say,, did you put the red wire back on? And if you did, did you turn off power to the furnace first? Don`t no what kind of furnace you have,, if it has a circuit/control board see if it has a push-in type automotive type fuse, check,is it good?, is all is good turn off power to furnace wait a minute and turn back on, sometimes this will reset the tstart.
Its possible the transformer might be bad

I need the wiring diagram for a Simple Comfort 220 thermostat.