I've NEVER had bump starting work on either of my e30's, its also warned against in factory manuals. When you put in this new battery do you KNOW it was at full charge? (like check it w/a volt meter) Needs to be over 12.45 volts or its not fully charged. When you turn the key do you get any type of "clack" or clicking noise? Do you have a kill switch or alarm system? Are the battery terminals clean and tight? Aside from sitting has anything changed from the last time you started it (maybe a rodent got up in the engine bay and chewed the wires)? Check the basics first and find out WHY its not working, running one of these cars with no battery can fry the ECU, all kinds of stuff.