94 Honda Prelude will not shift into reverse. No recent modifications. Fluid levels okay

Check brake fluid level in the clutch master cylinder reservoir. If that doesn't solve the problem, it could be a problem with your clutch slave unit, clutch, or synchros. Several people I've talked to have had problems caused by the lubricant they've used adversely affecting the yellow metals used in the synchros. I don't recommend used regular 10W-30 or 10W-40 engine oil in Honda MTs any more. It's best to use manual transmission synchromesh fluid, such as Amsoils 5W-30 synthetic MTF product.