[jpgvm]

It ran out of fuel. There was enough to relight the centre engine but not the 3 outer engines required to complete the landing.

[GraemeL]

It didn't actually run out of fuel. It didn't have enough of the hypergolic mix used to ignite the engines (triethylaluminium & triethylborane that causes the green flash sometimes seen at ignition) and only one of the three engines needed for the landing burn re-lit.