1996 caravan,suddenly no reverse

My friends 1996 caravan suddenly has no reverse,went to the mall worked ok,came out and tried to back up,and it just revs like neautral.There was no gradual loss or noise.All other gears work ok,and there is lots of fluid.I thought if the bands went there would be some warning signs of wear,or gradual problems.Is there any type of switch or solenoid that could have failed to cause this sudden loss,or any other ideas? .Thanks.

3 or 4 speed transmission?Possibally a busted planetary set and it is a common problem.Pull the pan off to see if there are any metal shavings and that will tell.