My old thermostat only had 6 wires. But according to the diagram I need 7 for the Robertshaw 9620. I don't currently have one connected to C... and it won't turn on at all. Any suggestions?

if you don`t have a wire for c===common, it will have to have a battery

did you note what each color wire went to what post on the old tstat? Very important

Yes they were all color coded on the old one. Now there is a brown and gray wire that were not hooked up to the old one. Wasn't sure where those would hook in or even if they do.