What do I do if I get deranked in an unfair competitive game?

What can I do about this?

Unfortunately, the only thing you can really do about this is to report the player for cheating. There is nothing much a common player can do about these cheaters except to report them for cheating.

However, I would like to remind you to be absolutely certain that your opponent is cheating before doing so.

The developers of CS:GO have tried a multitude of ways in which to weed out cheaters. Most notably, is the existence of CS:GO Overwatch. see the link for more information.

In conclusion, while you can't expect to get your opponent banned for sure, you can be certain that if he were cheating, he will be caught sooner or later. Justice will be served.

How about my rank? Can I get it back?

In the case of an opponent being banned, after being reported for that particular match, the match would be considered null and void. It would no longer carry any weight in determining your ranks.

Party members of your opponents and your entire team would have their ranks and ELO points adjusted to their appropriate value.

In short, should that player be found cheating in that particular match, yes, your rank will essentially be "reset".

Just a friendly tip...

I would like to end off by telling you to expect to run into cheaters often. The CS:GO community has always been littered with them. In your case, you are a mere Gold Nova 1. When you reach the higher ranks, especially above Legendary Eagle Master, cheaters are abundant every 3 or so games.

Take it from me I have been there and all I can say is, half the time, I would just have to live with the fact that those players are likely to go unpunished.

Even if you were to lose your rank and not get it back, do not be disheartened everyone faces cheaters but eventually, we all get the rank we deserve.