What do you mean by spun a bearing?
A spun bearing is the kiss of death for an engine. It essentially renders the engine block useless.
The crankshaft has a series of machined sections along it's length called journals. These journals rotate in "main bearings" which are contained in pedestals (bored into the engine block casting) on the top, and caps (bolted to the pedestals) on the bottom.
The journals never actually make contact with the bearings. They ride in a thin film of oil on the bearing surface. That film of oil is supplied by the engine's lube oil pump via internal bores in the engine block.
The main bearings are formed by two, serviceable semi-circular bearing halves (called inserts). An insert is made up of a steel shell and a coating of softer metal applied to the ID (called babbit).
These two insert halves are designed to be stationary. They are held in place by a locking tabs on the OD of the shells which fit into notches in the pedestals and caps. This shows how a bearing shell is held stationary within a pedestal. Notice the locking tab in the shell and the matching cutout in the block:
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There are basically two types of engine failure related to bearing damage - but there is a big fundamental difference between the two.
One, the least harmful, is when the bearings are "wiped".
The other, which basically renders the block unusable without extensive machine work (i.e. line boring) is when the bearings are "spun".
If an engine is operated without a sufficient supply of oil to its bearings - even for more than a short time - the oil film between the rotating journals and associated bearings overheats and breaks down.
Without that film of oil to separate the rotating steel journals from the stationary surfaces of the bearings, metal-to-metal contact quickly begins to take place. Depending on how long this contact is allowed to continue, the babbited surfaces of the bearings will heat up and melt. Eventually the melted babbit will start to be wiped away by the motion of the rotating journals (hence the term "wiped bearing").
At this point the bearings are ruined but the engine can be salvaged. The bearings are designed to be replaceable without any machine work. Whether the crankshaft is reusable as is, needs to have the journals surfaced, or is just trash - is determined by the degree of damage that the journal surfaces suffered.
Now comes the worst case - spun bearings:
If the oil film is lost and metal-to-metal contact is established, damage to the bearing and journals begins immediately. If the engine is not stopped (or by some miracle, oil flow is re-established) the bearing and journal soon become so hot - due to friction - that they will actually be welded together.
When they are welded together, and if the crankshaft continues to rotate, the bearing shell breaks free from the machined grooves in the bearing cap/block and begins to rotate along with the journal. When this happens, the bearing inserts are literally spinning within the bearing caps/block bore (hence the term "spun bearing").
If the bearings inserts spin within the bearing cap/block bore for even a brief time, the back sides (of the bearing inserts) will grind against the block and alter the dimensions/seating surface of the bore.
Point of all this is that - if you haven't operated the engine without sufficient oil for too long - there is a chance that you may not have spun any bearings. If that is the case, the engine can be rebuilt without any machine work.
