My 1989 300TE instrument cluster stopped working. The only that works is the economy gauge, speedometer and clock.
What should I start looking for?
The fuse looks OK.

Check fuses. If you removed the cluster recently, perhaps the 15-pin plug fell out. That's for your oil pressure, fuel level, engine temp, left bank of annunciator lamps, and illumination (did I miss anything )

I had tried to remove the instrument cluster like 1-1/2 yr ago but then
I did not as my odometer is broken.
I left it alone.
Then, suddenly this happened.
I will remove it tomorrow and see if I see the 15 point connector.
Will check the fuse before I mess with the instrument cluster.
Thanks

I forgot to mention one thing. If the tach works, this might be a fuse problem, because the tach is powered by a separate connector.

Sounds like Jer is leading you in the right direction with the fuses, you may want to replace all the fuses simply because of their age. With the fuse(s) removed push the spring loaded tabs downward, this will help increase the tension placed on the fuse tips..