Why won't my PC power up?

My Medion MT7 MT448G desktop PC won't power up, having stopped in use. Have done the obvious (checked power lead, fuse etc.)
Initially changed the PSU for a 235W spare one I had, & the main processor fan whirred to life, but PC wouldn't boot up - then I realised that PSU was missing an extra 4 way connector (2 YW, 2 BK, = +12v?).
Have now changed the PSU for 400W ATX version with this additional 4 way lead, + an extra 4 way lead (RD, YW, OR, BK)that clips onto the end of the main 16 way plug.
Now, NO boot up & even the processor cooling fan doesn't come on!! Any ideas, anyone??
Thanks.