If you received this email, no - Monument did not get hacked.
Instead, another website you use suffered a data breach. This other website had their users' data stolen and released online. You are at risk because you used the same email and password combination on the other website as your Monument account.
We send this email so you can change your Monument password to keep your account safe. Malicious actors often take the email and password credentials from a data breach and try them on various websites for bad purposes.
Remember: it’s always best to use different passwords on different websites.