You have two chances here, and they both end in "all".
To solve your problem, have a close look at the wiring to the thermostat. Two should be marked "TH" and two should be marked "90". Using your test meter, two terminals should read damn near zero ohms, and two should give a reading in Kohms. The wires marked TH should be fitted to the high resistance terminals, the ones marked 90 to the low.
That's probably as good as it gets, I'm afraid.
Next time, diagram or photograph, before doing anything.
Power off, of course.
Which bit of "Unplug it" is causing the problem?
Stop eMailing me. This is a perfectly good forum - use it!