First thing, start the engine and pull the plug wire off of the offending cylinder. Do you notice a change in the engine speed or smoothness. If not, ground the plug wire to the block and see if you can hear a spark jumping from the plug end of the wire to the block. If not, you have no spark to that cylinder. Easy fix. If you hear a spark jumping, you have spark to the cylinder and your problem is more complicated now.
Using any
oil lately?
Smoking a little from one side or the other?
Audible missfire?
Time for a compression test.
Oil is possibly creeping past rings or valve guides.