Do you have a site plan from your ownership certificate or a cadastral plan? That's the most important thing, because without documentary proof of the boundaries, any photos and complaints will just go nowhere.
I agree that you should take photos, but that's just backup. What really saves you is a cadastral passport or an extract from the Unified State Register of Real Estate - it clearly shows the coordinates of your plot. If the plan matches where the fence originally stood, then you've got a strong hand. You go to the administration with this document and file a complaint about unauthorized land seizure, and they're obligated to investigate. They can even hold your neighbor liable administratively.
Here's the catch though: if your plot was never officially surveyed, and the fence just stood there "as it was" for years, it's harder to prove the exact boundaries. In that case you might need to pay for a survey yourself - that's money and time. But even that's better than silently losing two meters of land.