Last night simulator was flying as usually and suddenly – Wham! – it lost all hydraulic power. “Low Oil Pressure” was indicated in Display of Motion Electronics Cabinet. It was flying normally for 20 minutes after Motion and Control Loading pumps were re-started, but goes down again.
After that I checked actual oil level of HPU. It was pretty low and causing pumps stopping. I had no option but add some hydraulic oil.
A lesson was learned: even small drops of oil leakage will result in low oil level if level is not checked regularly.