Hydraulic system failed and would like to know how to fix it. Agco ST25 wit?

The hydraulic system failed on my Agco ST25 tractor with loader. I would like to know what to trouble shoot to find the problem and hopefully repair myself. The hydraulic fluid was a little low but when topped up the hyd still did not work. The tractor is Hydrostatic drive as well if that makes any difference but it still drives ok. Hope I can get some feedback and thankyou in advance for any guideance you can give me.

Roy