Most likely reasons;
- You are over your daily query limit see Google API information. If you are on a shared Host this can impact those limits.
- You moved your site or changed the domain name and did not change your Google Map API key. See Getting Started
- If you have the Power add-on using the import function and your csv file is incorrect, or your header rows are not accurate, or they have wrong field names, the columns may be misaligned and the data field for an address could not import and/or does not geocode. Check your import message log.
- Google does not recognize an address. See FAQ post