My Phone Has Been Stolen

Hello,

No. The only way that someone else could gain access to your iCloud data is if they know your Apple ID password. If you have two-factor authentication turned on (you probably do), then you would get notified when anyone else tries to access your data:

If you think your Apple ID has been compromised

Two-factor authentication for Apple ID

Good luck.

Hello,

Yes, on another device, use the Find My app. See this for detail:

If your iPhone, iPad, or iPod touch is lost or stolen

Good luck.