No, dirt is generally not an issue. At 20 years, a defective
therocouple is quite reasonable. They are easy to install (screw-in
deal), but considering your lack of familiarity with gas appliances,
I'd call the gas company, or whoever services your burner. The other
issue is making sure that you have the right size themocouple. Not a
big deal, and 1/2 time, at most. It is also possible that you have a
defective gas valve, or power is not getting to the electrical
controls on the valve. Check fuses first, before you call, and that
the breaker is Ok for that circuit.