1995 BMW 325 not starting

I dont know what to do anymore I need help. I was driving my 1995 bmw 325i and it was working fine, I came home in the afternoon and it was running fine. The next morning I tried to start it and it would try to start but failed. I left it alone for a month because I dint have the time and money to fix it because I have another car so I dint pay much attention to it. Now that im on vacation im trying to get it fixed. It wouldnt start so I gave it a jump and it started which it didnt do before. But when I turned it back off and tried turning it on it wouldnt start. I changed the battery and it still does the same problem. I thought it couldve been my starter but it seems to be in working condition according to my mechanic and the fuel pump seems to be working to. What could be the problem, can you help me in figuring out whats wrong with it. Oh and I had the o2 sensore changed 2 weeks before the problem but where I took the car they told me I needed to perform a fuel injection cleaning but I didnt trust their opinion because they looked a little shady and I regretted taking my car there.