@Odie;
Yes, Onslow is correct...the D70 ONLY allows mirror lock-up for cleaning purposes.  He is also correct that manually poking at the mirror can cause focussing problems...so, be sure that the clerks "fix" has not caused you more problems.....

Technically, the firmware will NOT allow you to release the shutter button once the battery gets too low....been there, done that....so, I don't think it was a battery issue.

A few things to note;

1. Make sure you have the latest firmware on the cam
2. If you have any quirky things happen, it sometimes helps to do a camera "reset" to unscramble it's brains...check your cam's manual regarding the reset function...

My D70 is 3 years old now....and have taken oodles and oodles of shots with the SB800....no problems so far.

Hope your situation was just a "camera having a bad day"....

In youth, we learn....with age, we understand.