There are several safe sites offering a
trial version of the software but you need to fork out $34.99 to keep it working after the trial period expires. They include Download.com (a.k.a. CNET) and Softpedia. (Both of those sites are 100% safe as long as you make sure that you read everything carefully during the installation process and deselect anything you're offered but which you don't want).
However Microsoft never offered the program for free (unless, possibly, it was included with pre-installed software on a new PC) and there can be no guarantees that it would now install properly on a Windows 10 machine.
Even so, software doesn't usually disappear from a computer. It's far more common for the shortcut(s) on a computer's desktop, taskbar or Start menu to vanish. So dig around a bit on your hard drive, especially within the folder called 'Program Files (x86)'. If you can find the executable file for the program, double-click on it to see if will open normally. If it does, go back and right-click on the file. That will bring up options to 'Send to > Desktop (Create Shortcut)', 'Pin to Taskbar' and 'Pin to Start Menu', so that you can choose the one(s) you want to make use of.
If you have to give up, and want a decent alternative, consider the following freebies:
GIMP is a fully-featured image editor, akin to the mighty Photoshop. It's got a massive learning curve but it can produce professional-standard results:
https://www.gimp.org/
Irfanview is mainly known as an image-viewing program (rather than as an editing program) but it's got lots of editing features built into it if you look 'under the hood':
https://www.irfanview.com/
Paint.NET has lots of fans among AB members:
https://www.getpaint.net/