The Wikipedia article on STS-1 (the first orbital space shuttle mission), in a discussion of all the problems and malfunctions occurring during the mission, mentions that, due to an unexpected shockwave during booster ignition, Columbia’s rear bodyflap (part of its pitch control system) was (unbeknownst at the time to either Mission Control or the crew) bent upwards beyond the point where damage to the shuttle’s hydraulic systems would have been expected.
According to the article, had Columbia’s hydraulic lines actually been breached, resulting in a total loss of hydraulic fluid, this would have rendered the shuttle uncontrollable as soon as the aerodynamic forces on the orbiter during reentry exceeded the control authority of its reaction-control system.
Small- to medium-sized aircraft with hydraulically-actuated flight controls essentially always have some form of manual reversion capability in case a no-hydraulics situation occurs, where the primary flight control surfaces (always the elevator and ailerons, and sometimes the rudder as well) are operated manually by the pilots, usually by control cables attached to the pilots’ yokes and pulling (depending on the aircraft) either on the control surfaces directly or on servo tabs which then generate aerodynamic forces which move the control surfaces. (Large aircraft generally don’t, even though even partial control authority would still be better than nothing, but that’s another question.)
To the best of my knowledge, the largest Western aircraft to have flight controls with manual-reversion capability is the Boeing 737 MAX,1 which has provisions for full manual control of the ailerons and elevator, plus a small amount of undocumented manual rudder control (as the rudder is rarely or never used during normal flight, manual rudder control is far less critical to have than manual elevator and aileron control, and using a manual aileron-and-elevator system rather than a manual three-axis system allows for a simpler system; however, due to the design of the 737’s rudder system, the rudder can nevertheless be manually moved slightly by a sufficiently large amount of force on the rudder pedals). The 737 MAX is considerably larger than a shuttle orbiter, with a maximum weight of 88,300 kg, as compared to the shuttle’s empty weight (as its flight control capability, or lack thereof, would only be relevant during descent, when it would generally be empty, and, therefore, at its lightest) of 68,600 kg; even with the shuttle orbiter at its maximum allowable weight (to allow for the possibility of a hydraulic failure during an abort scenario while carrying the absolute heaviest possible payload) of 109,000 kg, it would be less than a quarter again the size of a 737 MAX 9, which would seem to indicate that including manual-reversion capability for the shuttle’s flight controls would have been doable.
Why, therefore, wasn’t it included?
1 The Ilyushin Il-62 is almost twice as heavy as the 737 MAX 8, with an MTOW of 165,000 kg, and has a fully mechanical flight control system (no hydraulics even in normal operation); however, it was a Soviet aircraft, making it somewhat unlikely that the space shuttle design teams could have gotten access to its technical data (especially given the heavy involvement of the U.S. Air Force in the design and development of the shuttle - for instance, the shuttle would have had much smaller wings were it not for the design requirement to launch into a polar orbit to either deploy a U.S. spy satellite or kidnap a Soviet one, and then reenter and land after just one orbit, with the consequent large crossrange requirements).