You may have heard about an issue with Lookups not working correctly with a recent version of Chrome (v38). Microsoft documented the issue here in a KB article earlier this month.
Last week we noticed that in a new CRM Online trial org with a version of (188.8.131.521) (DB 184.108.40.2060), the Chrome v38 Lookup issue was resolved so it seems that Microsoft was working on a patch. Today though we have noticed that with the latest version of Chrome (v38.0.2125.111) the Lookup issue is fixed across the board in CRM Online, CRM 2013 On-prem and CRM 2011 On-prem.
So if you or your users were stuck on v38 of Chrome then be sure to go to Settings –> About and make sure you’re updated to the latest version to fix the Lookup issues.
To check your CRM Online version, click the Gear icon at the top right and then click About.