All preview sandboxes were updated to Spring '19 last weekend and the first round of production orgs is getting updated this weekend.
We have been working with Salesforce on an issue with the Spring ‘19 release wherein a blank page displays on the page if you have ALL of the following in play:
Enable clickjack protection for customer Visualforce pages with EITHER standard headers or headers disabled
A My Domain name that contains a capital letter
Using VisualForce pages in Lightning Experience
If the above scenario describes your org, please whitelist your My domain name via Session Settings as explained in this documentation. Please note that only Spring '19 orgs have the option to whitelist domains from Session Settings.
As always, we encourage you to try this out in a sandbox environment first. If you continue to have issues with this after whitelisting your domain name, please let us know and we will continue to investigate.