34:19 is giving you good advise (as usual)
Heres another idea.
There is a valve between the crankcase and oil tank that stops oil from returning to the crankcase while the engine is off.
It is probable that sitting so long allowed all the oil in the oil tank to return to the crankcase, emptying the oil tank.
It's possible that once you started the bike, all that oil is getting pumped (slowly) back into the oil tank replacing the air with liquid, causing the pressure you felt.
If you have even a little oil in the tank (use a light, don't add some) I would start the engine for ONE minute, listen for bad noises, shut off and re-check the oil. Is there more oil? If so, I'm right, start it again and let it warm up 4-5 min or until the fan starts. Then re-check the oil level.
The oil needs to be warm, and in the tank to drain it affectively anyway.