In essentials there is no way to fix a line when this one ends up square by any reason, for example if you erase part of a line you can’t make the tip round again.
But as I said and how I show it in the links that I provide, you can find you in a situation were you just need to “reset” the tip of the line and you can not do that which cal leave you with lines with “cuts” in the middle.
You could move the two line parts closer to each other and then flatten them.
Or you could zoom-in and add a connecting line, or you could “close gap” (alt+c)
if you need a closed region. If the line is wide or has ends dangling off you could
erase those parts.