My 2002 frontier will not go in reverse and only goes 25 mph. no warnings.

Sounds like it's a failed transmission!

get a free diag from aamco.these are known for trans coolers leaking water into transmission and taking out the insides.nissan covers that to 100k miles i think. when they do this is also takes out the valve body which is expensive.did several in the past approx cost 5000.00