Dropped cyl 3 to no compression

When head was recently off getting guides, surfaced, etc quite sure the jugs were disturbed and opted to replace base gasket. After re-assembled, had steller comp across all four cyls and  even rode it around the hood a little.
Then, the other day fired it up and soon found myself chasing a dropout on cyl 3 issue to find zero comp in that cyl. 
For what reasons might this just so suddenly occur after having ran good for over an hour already?? Thanx

 

Could be over a million things.
Check your vavle lash.
If you have a boroscope, pull the spark plug and look inside the cylinder.
While looking inside with the scope, rotate the crankshaft with a ratchet to see if the valves are closing completely.

My guess would be head gasket given you've had the top end apart. Did you torque the head correctectly then retorque it after a run. Any signs of blowing when it's running. If it was a valve you'd hear a god awful racket when it was running. 

No I heard nothing, was good for a test run getting it to temp, re-torqued but was only able to smidge a few of 'em,
then days later had idled it for 5 mins while fine tuning DS-2 modules with old craftsman adv adjustable timing light. Ran and sounded great!
Then started and right out the gate was a cyl down, 3rd pipe was cold so pulled plug checked inside with light and nothing apparent.
Pulling it apart to find out right now, so we'll find out for sure. Just weird, that's all. 

I really suspected a bent valve so pulled the head last night and all cyl's held water overnight, not a drop leaked!! I did a good job on the head work over the winter months, really just took my time on it. So far so good