Blank screen for PM's and contact us

[birdie]

I still using v3.6 (don't ask!). I just sent a mass PM to all members (it was our birthday) and it appeared to work find. I got several replies from members. Now when anyone clicks on the PM link or the contact us link, they just get a blank white page? (everything else is working fine). The affected pages are sendmessage.php and /private.php

Anyone have any ideas.