Could you please check if the settings disable email tracking for all users in your account was disabled recently? That could cause the tracking to not be working.
Also if it's not active, my recommendation would be to please remove and install again the extension to see if this issue persists.
HubSpot sales email tracking can sometimes be deactivated if the recipient's email client doesn't allow the tracking pixel to load. HubSpot tracking can be deactivated in the following situations:
The recipient formats their email in plain text.
The recipient's email client doesn't automatically download images.
The recipient's corporate filter strips away all images for incoming emails.
If the recipient has a very strict spam filter where they block any images or link tracking from loading in emails, your tracked email could be filtered as spam. However, email tracking tools are common, and the majority of email clients will not view your email any differently than before.