About a month ago I got a judment on my favor, plaintiff never show up to trail, now they reopen case, can they do that?

Sure... they probibly proved they were not properly served notice as one example.

There are many ways and reasons for this to happen.

Did they move to re-open, or has such a motion been granted?

If it is the former, you should oppose. Also, depending on your jurisdiction, the court rules will often provide that they only have a certain amount of time in which to ask to re-open the case.

Any question on law needs to include your general locale as laws vary by area. Also you posted this under Small claims and the rules for small claims courts are more relaxed and vary from court to court.

You said; "now they reopen case", but that's very vague. It could mean they filed a motion to reopen or the court granted a new hearing or somewhere in between. They certainly can move to reopen, whether it will be granted and a new hearing ordered depends on their grounds for reopening. Information that you did not tell us. Please, if you want help from us, you need to give details.