Cessna 172 low performance at 6200 FT

Today I made long flight thru KTVL airport (6268ft height) where I fully stopped and shutdown the engine. After short break I started engine again.
I tested performance, but engine was able reach 2000 RPM only - with full rich mixture and even with 35-40% rich mixture.
When I start Cold & Dark at KTVL again I can reach full power.

Can anybody tell me what was wrong? What I forgot turn on or off please?

Thank you.

Moved to #student-pilots:basic-gameplay-help

Can it be the icing?
How to de-ice in classic Cessna 172?