If the switch is set to MF and the AF motor is engaging there is something wrong with the lens.
- The specific lens model and how much it is worth versus how much it will cost to have it repaired
- Whether it is still covered under warranty
- Whether it might have been damaged in a way that would preclude warranty coverage even during the warranty period
- Whether you are willing to use 'back button' AF to work around the issue
You can either decide to have the lens repaired, you can decide to replace the lens, or you can decide to use the lens as is and work around the problem using custom camera settings to separate AF from the shutter button half press using 'back button focus'. For Canon cameras that do not have an 'AF-ON' button, you can use the menu to set the 'AE-Lock' button, the one with the little six sided star [*] next to it, to act as an 'AF-ON' button.
It is a Canon 50mm f/1.4 I believe
If it is indeed an EF 50mm f/1.4 (hey, if you aren't sure how are we supposed to be?) there is an additional thing to consider: The fairly recent EF 50mm f/1.8 STM. For more comparing the EF 50mm f/1.4 USM and the EF 50mm f/1.8 STM (as well as the EF 50mm f/1.8 II, and the EF 40mm f/2.8 STM), please see the following part of this answer to this question.
Having said all of that, if I were looking for an economical 50mm prime today I would carefully consider the new EF 50mm f/1.8 STM. It is just as good optically as the f/1.4 and f/1.8 II. A lot of the build quality and usability problems of the f/1.8 II have been well addressed by the design of the F/1.8 STM. It sells for a fraction of the price of the f/1.4. The one thing that might be a downside for some users is the way the manual focusing ring works with all STM lenses. The focusing of the lens, even when manually focusing, is strictly focus-by-wire. Turning the focus ring on the barrel of the lens does nothing mechanically to move the focus elements of the lens. Instead, an electrical signal is sent to the camera body which in turn sends a command to the focus motor in the lens to move. This limits the smallest increment of focus adjustment to one "step" of the stepping motor in the lens. It also eliminates the ability to focus the lens when using "dumb" extension tubes or freelensing. You can't move the focus elements to park them when the lens is not attached to a powered up EOS camera body either.
You can also check out:
Canon 50mm f1.4 no longer focuses reliably
STM vs Non STM lenses for still photography?
Why do higher end lenses use USM instead of STM? (Keep in mind that the AF motor in the EF 50mm f/1.4 USM is not a true ring USM, it is a so-called "micro-USM.")
I recently encountered an EF 85mm f/1.8 that needed some internal cleaning (well beyond dust that does not affect the image). In the process of disassembly/reassembly the plastic prong of the actual electrical switch beneath the external AF/MF switch was damaged. It can be moved, but the lens must be partially taken apart to do so. The prong is no longer long enough to engage the guides on the back of the external switch that sits on top of it. I chose to set it to AF and reassemble the lens. Since I use back-button AF most of the time anyway, if I want to manually focus the lens, I only need to not press the 'AF-ON' button before taking the photo. The EF 50mm f/1.4, EF 85mm f/1.8, and EF 100mm f/2 all use the same basic construction including the same less than elegant interface between the external mechanical switch on the lens' exterior and the actual electrical switch underneath it that is directly soldered to the main PCB. It's very easy to damage the switch when assembling the lens. That might be what happened to your EF 50mm f/1.4.