Running 12.04 LTS with Windows , Ubuntu doesn't boot. Have tried to re-load sw but Windows seems to block it

Running 12.04 LTS with Windows , Ubuntu doesn't boot. Have tried to
re-load sw but Windows seems to block it

Running 12.04 LTS with Windows happily for years, Ubuntu doesn't boot now.
Have tried to re-load sw but Windows seems to block CD disc. I'm only a
simple user with no programming skills. Have loved Ubuntu but now can't
get my machine past appalling Windows. Help? Bootloader doesn't work
either.