I lost my MacBook so can they hack in and delete my iCloud

See the following from If your Mac is lost or stolen - Apple Support

1. Change your Apple ID password to prevent anyone from accessing your iCloud data or using other services (such as iMessage or iTunes) from your Mac.
2. Change your passwords for other accounts you use with your Mac, including email, banking, and social sites like Facebook or Twitter.
3. Report your lost or stolen Mac to local law enforcement. They might request the serial number of your computer. You can find this information on the original box or receipt you received when you purchased your Mac.

I'm not sure what the problem was but if you haven't already, change passwords as described in items 1 and 2 of my previous reply.