Did you double-check the date range to make sure you were looking at the exact same date for both reports? If so, check the timezone settings for both accounts to make sure they are both set to the exact same timezone setting. If those are correct then you may need to look deeper to find the cause.
Check your web server logs to see if there are errors. Perhaps your website went down for a period yesterday while you weren’t paying attention to it.
Check that your Google Analytics implementation is correct, by installing the Google Tag Assistant tool for Google Chrome and check that the implementation shows green on all pages that you use for a lander. There could be an issue that causes some traffic to not be counted.
Compare Sessions vs Users within Google analytics, if you have a lot of sessions but few users then you have the same person, or persons making repeated visits, that could account for the discrepancy, though it is much larger than it should be unless something dubious is occurring.
Your website may have been hacked. Scan your website for scripts that might be redirecting traffic. If you are using WordPress with plugins or themes from sources other than the wordpress.org repository check those files for hidden scripts, this is a common issue.
Inspect all of your ads for proper final URLs. Double-check all landing pages for proper functioning when you click the link in the ads. Check to make sure that no impressions came from ads that are now disabled or removed from your account (can occur if your Google account has been hacked).
If none of those things help you resolve the issue, contact me via PM and I’ll take a deeper look for you.
