Why all of a sudden can't i go to the linked invoices from the purchase order?

I'll take it as you're referring to the hyperlink when creating Bill from Purchase Order (P.O.), accounts237.

We have an on going issue about this one. Rest assured, our engineers are still investigating why the link to create a Bill from P.O. has disappeared. 

In the meantime, we'll have to open the bill to see the purchase order associated.

I'd also recommend contacting our Customer Support team so your company file will be added as one of the affected users. Here's how:

1. Click the ? Help icon, and then click Contact Us.
2. Enter I can't go to the linked purchase order from the Bill page.
3. Hit Let's talk.
4. Select Start a chat.

You can contact them every Mondays to Fridays from 6:00 AM PST to 6:00 PM PST. Then, every Saturdays until 3:00 PM PST.

I've also added an article for some frequently asked questions about purchase order: Create purchase orders.

I'm always right here if you need my assistance.