Hi Sarah,
What mbrsolution says is right, but let me elaborate a little more.
Web hosting companies set memory limits for websites to use at any one time. The error message is saying that your site has reached its limit.
41943040 equates to 40 Megabytes, which according to our statistics puts you in the bottom 0.2%. i.e. Only 0.2% of web hosting companies have such a low limit - 99.8% have a higher one.
Solutions:
1) It may be possible to raise your memory limit, as in the thread that mbrsolution links to - add this to your wp-config.php file:
define('WP_MEMORY_LIMIT', '64M');
2) You could ask your web hosting company to either raise the limit, or justify it. There's really no justification for it though - as I say, they're easily in the bottom 1% there.
3) More technical is to go through your plugins one-by-one to find out which is the one using up most memory. Just because UpdraftPlus tips it over the limit doesn't indicate that it's the one using most memory - it just means that when UD gets its turn to run, it uses more memory than what's left of the 40Mb; but "what's left" at that stage could be anything. You can try disabling all your other plugins, and looking at the UpdraftPlus page, in the "expert" section (down the bottom) - there, it has a counter of memory used. Re-enable your plugins one-by-one and see which one causes the biggest increase in usage. I'd not really recommend spending huge amounts of time on that, though - it'd be more efficient to pursue 1) and 2) and if they won't raise it for you, switch to a new web hosting company.
David