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Which was the first liquid non hypergolic engine to be reignited in space? ( space = above 100km )

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It would seem to be the RL-10A used on Atlas-Centaur. According to a NASA document "Successful Restart of a Cryogenic Upper-Stage Vehicle After Coasting in Earth Orbit."

Atlas-Centaur AC-9, launched on October 26, 1966, was the first vehicle using cryogenic propellants that successfully demonstrated an engine restart in space.

Unless there were any prior non-cryogenic, non-hypergolic restarts this would appear to be the first.

Atlas-Centaur model
Atlas-Centaur model (National Air and Space Museum)

In case like me anyone thought the J-2 engine used on the S-IVB may have been first, the first in-space restart of the J-2 was a year later on November 9, 1967 on Apollo 4, which was the first test flight of the Saturn V rocket. The initial burn of the J-2 on this flight put the S-IVB and an uncrewed Apollo test capsule into a 115 mile circular orbit. This was followed by a restart of the J-2 which placed the capsule into an elliptical orbit with an apogee of 11,000 miles, and a resulting reentry speed which was similar to a lunar reentry.

Interestingly another NASA document stated:

The S-IVB on-orbit restart was the first restart of a LOX/LH2 propellant engine in zero-g.

Which does not seem to be correct. However this was just the text summary for a NESC Academy video (NASA Engineering and Safety Center), and apparently was based on a statement made by the presenter in the (quite interesting) video which detailed the drop tests that were done from the 365 foot dynamic test stand at Marshall. These tests (the video includes film from inside the test tank) showed that the ullage motors on the S-IVB were not effective by themselves in reducing fuel slosh. This led to the installation of baffles in the tank. At 16:10 in the video, presenter George Hopson, who managed the tests, stated:

Before Saturn V you never shut down an engine and then restarted in orbit, until you got to the S-IVB stage where you had to go around a couple of times before you went to the Moon, to check out all the systems.

Perhaps he was referring only to within the Saturn program.

Interestingly the Atlas-Centaur AC-9 test flight was directly tied to the development of capabilities for the Moon landings. According to the summary page for a book of compiled NASA documents titled "Atlas-Centaur 9", part of the America's Finest Rockets series:

AC-9 was the last test flight of Atlas-Centaur. Following the successful two burn mission, the rocket was declared operational for both 1-burn (direct ascent) and 2-burn (parking orbit) flight plans for delivering Surveyor to the surface of the Moon.

The first operational restart of the RL-10 was on the Surveyor 3 mission in April 1967 (which still beat Apollo 4 by several months). This was the first Surveyor mission to begin with a parking orbit around Earth, demonstrating capability which would be needed for the Moon landing missions. Somewhat fittingly Surveyor 3 was the vehicle visited by the Apollo 12 crew in 1969.

Surveyor 3 Pete Conrad inspects Surveyor 3 (NASA)

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