1999 Sable, was making knocking noise - now won't start

Need help. My 1999 Sable was making a knocking noise at first. I changed oil, added prolong lubricator, but it didn't help. Took it to mechanics they said the main bearings need to bee replaced, because the oil is not reaching the top. Now as I was trying to get it to the shop it wouldn't run. I can hear it click, trying to ignite, but it would only start and run for 20 sec and slowly die out on its own. After one trial it wouldn't start again. If I let it sit for a while it would run again, the same story - dies in 20 sec.

Any ideas as to the knocking (rattling) noise and the dying out will be most appreciated.