Without any viable information to work with, it sounds like your unit is going into a safety lockout. This condition could be a result of any number of reasons, and by powering it off and back on, that action alone resets the control, allowing a restart.
You need to have someone qualified to come and take look at it, ASAP.
It could be very minor, like cleaning residue from the flame sensor, or something more serious and potentially harmful.--
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