Airstarts have many bad possibilities
I couldn't agree more with both Ramen Noodles and Havoc. Have lost the engine in a Bell 47 below 500 feet once, so no time to even think about restarting but if the same had occurred at several thousand feet, I would have concentrated first on ensuring I could make it into a safe landing area before even looking at trying to restart.
I would only attempt a restart if I was aware of the reason it had failed - as RH & H have mentioned already. If you don't know what caused the problem and succeed in restarting the engine, you may be lulled into a false sense of security and continue the flight in the belief that all is OK now, only to place yourself in a worse position when the engine fails for the second time..... or catches fire etc........
Have had to restart an engine in a twin, but that is no real sweat unless you haven't assessed your performance correctly and can't maintain altitude on the remaining one!!