TH400 transmission not shifting

[pantera928]

Jumped in the usually reliable 1980 Avant today and started off to run some errands. THe car would not get out of first gear no matter which forward position I placed the shift lever in. It would go into neutral, reverse and park without issue. I drove it home and later went out and checked fluid and linkages and found nothing wrong. Took it out again and everything worked fine. 

Any ideas?

Thanks