Wait 24 hours. MultiUp automatically re-uploads dead files to new hosts periodically. Alternatively, use a debrid service (Real-Debrid caches files, so even if the original host dies, their server still has it).
If you cannot install software, sites like or deepbrid.com act as a proxy. You paste your MultiUp link, and they download the file from the best host to their server, then give you a direct download link.
To download files hosted on MultiUp, you typically have two options: using their web interface or a dedicated download manager.
If you want to automate your downloads, here are the best software clients.