The Ludendorff rail bridge was the last bridge over the Rhine at the time it was captured, and thus of great strategic importance. But let's not delude ourselves. Even had it been blown to cinders and ash, the Rhine would have been breached. The German Wehrmacht was very weak by March 1945, while the western Allies, especially the Americans, could draw on a wealth of experience and materiel. A nation that has successfully invaded dozens of islands and a fortified continent is not going to be given too much trouble by a single river.

We're getting into hypotheticals here, but given that airborne troops, amphibious tanks, and assault craft were used heavily in Operation Plunder a scarce three weeks later to enable the British to cross the lower Rhine, we can pretty safely assume that those three tools would have been used to establish a bridgehead had the Ludendorff bridge fallen. Once the far bank was secured, a pontoon or Bailey bridge would be set up, enabling reinforcements and supplies to cross.