I changed my Apple ID, and lost all contacts?

Did you change your existing Apple ID email address or did you use a different Apple ID? If you changed the email address and were syncing Contacts with iCloud, have you allowed time for the contacts to sync from iCloud to your device/computer? Are the contacts showing at iCloud.com If you weren't sync Contacts with iCloud, do you have a backup you can restore from? Restoring from a backup erases all data currently on your device. Any data newer then when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup to see the contents and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.    *

ASC YouTube for how to restore an iPhone, iPad, or iPod touch from an iCloud backup

Did you change your existing Apple ID email address or did you use a different Apple ID? If you changed the email address and were syncing Contacts with iCloud, have you allowed time for the contacts to sync from iCloud to your device/computer? Are the contacts showing at iCloud.com If you weren't syncing Contacts with iCloud, do you have a backup you can restore from? Restoring from a backup erases all data currently on your device. Any data newer then when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup to see the contents and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.    *

ASC YouTube for how to restore an iPhone, iPad, or iPod touch from an iCloud backup

If signed in with a new ID, sign back in with the old ID and recover the contacts. When changing to the new ID, make sure you turn off Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on. There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. If possible, also set up Family Sharing  between the IDs to keep access to your apps/purchases. 

 How to switch Apple IDs on your iPhone or iPad.      

 What to do after you change your Apple ID or password - Apple Support.    

How to transfer all files from one iCloud account to another.    

Family Sharing.         

Family Sharing - Set up.