A bad hit can cause permanent hearing loss. They were looking for other issues with the CT/MRI scans that might need further physician follow-up, and it sounds like they didn’t find anything. Go see an audiologist to see what your options might be for managing your new hearing loss and tinnitus (are you really TOTALLY deaf as measured by a hearing test, or are you just feeling that way comparing that ear to your good ear?). With a bad hit like that, you may also want to look into whether you need follow-up in regards to any other possible post-concussion symptoms if you are noticing other things (e.g. sound and light sensitivity, dizziness, headaches, persistent head and neck pain). With a sudden dramatic hearing loss, it might also be worthwhile looking into grief counselling.
The whistling is an effect of the damage to the ear. It is not dangerous and it is not a red flag for anything else now that MRI has ruled other issues out, so you don’t need to worry about it being some sort of warning sign. There is a reasonable likeliness that it will calm down over time if you can manage not to focus on it too hard–but it will take much longer to calm down than you would like (my sudden onset tinnitus was very dramatic for about six months, and tolerable but regularly annoying for probably another two years before it really calmed down, for what it’s worth). Turn on some music to give yourself something else to focus on, get some sort of relaxing sound that you can turn on over night. The audiologist can help you with managing the whistling as well. If you do still have some usable hearing on that side, there’s a good chance that amplification (a hearing aid) will help with the ringing.
Let your loved ones know that you have sustained a dramatic injury that will have ongoing effects on your life. Losing an eye is more VISIBLY dramatic, but the people close to you need to understand that there is a similarity here. The most obvious difficulty (the one they will more easily remember, although they will still have trouble remembering and need a lot of reminders because it is an invisible injury) will be hearing people on that side, but your brain uses two ears to process speech in noise and you will probably notice the change in noisy social environments. Let them know that background noise will now effect your ability to understand them much more dramatically than it did before and you will need their help in finding the best ways to communicate with one another in those circumstances. You may also need to think about the effect that this might have in your workplace and look into acommodations. Again, an audiologist can help you with this.
There are many people wandering around the world with hearing only in one ear just living their regular lives, and in the long run you are going to be fine. However, there is going to be a period of adjustment. I think often when people lose the hearing in one ear they try to go right back to their life like nothing much happened, and get a bit of a shock down the road when they realize that 1) they are struggling more than they expected, and 2) they did not give themselves time to grieve. What you have lost is NOT insigificant, even though it is invisible. Gather your support system around you. Communicate this experience as best you can–it is easy for others to imagine what it would be like to lose an arm and empathize, but very hard for them to imagine what it is like to lose an ear.
I am very sorry that this happened. Take some time to process it. Take the steps you need to ensure you know what your options for management are. Find the people who can help you. And know that despite this change, you are going to be fine.