Blender Separating vertices on object when restarting Blender Summary:
I am having this extreme problem with Blender where I have this character I am creating. The character uses a mirror modifier, and I am using Blender 2.83. I saved my file and restarted Blender and when I restarted Blender, some vertices and edges were not together and made it's own separate object.
What I  Tried:
I tried everything with all joining of vertices with using the F key and all. I cannot find any answer on the web about this. I am just learning Blender, but I know that everything was together and I know I didn't press anything that could cause it so I desperately need help with this.
Image Illustrations

Image of what is should look like together:
Isn't actually together but what it should look like together.
This image below shows it together and should be connected with the whole surrounding, it worked before I restarted Blender.

Image when it is separated:
But once I restarted blender, it wasn't together like it is suppose to. I had trouble selecting the whole separate object but it still proves but point on how difficult it is when it is separate.

Also, while I already have this question written, how do I hide and show that selector mover thing, the green, red, and blue arrows one, I accidentally shown it but don't know how to hide it again. I do have some uses for it in the future hopefully but like to hide it in the mean time so it's out of the way.
I believe I found the answer to this. It took a while but I think I finally found it. In Blender 2.8, you will select everything by pressing the &quot;A&quot; button and then you will go to the Mesh option on the nav bar on the 3D screen and hover over the merge option and then click by distance. So if anyone has this problem then this should help. If my answer seem to be confusion then here is a screenshot.