Big companies that were meant to attend GDC 2020 pulled out of the event in large numbers which eventually prompted questions whether it would happen at all. GDC then officially revealed the event would be postponed.
GDC 2020 ran into trouble that was caused by a sudden lack of participants after Microsoft, Epic Games, Activision Blizzard and a pile of other big companies in the industry decided not to participate due to health concerns.
As you probably know by now, these health concerns were prompted by the outbreak of Coronavirus and considering that GDC 2020 was meant to be held in San Francisco, the risk of having large groups of people in one place was too big, despite no cases of COVID-19 exposure being reported just yet. Pre-emptive measures are, after all, the best way to counter the outbreak.
Anyway, while everyone expected GDC 2020 to be outright cancelled due to so many participants pulling out, the organisers decided to only postpone it. According to the official statement, they still "fully intend to host a GDC event later in the summer" which would be favourable for the enthusiasts.
Hosting or postponing an event of such scale is a matter of its own though and there are currently speculations that the sheer amount of effort needed to organise it again is the reason why the official GDC postponement announcement came so late.
There is also an FAQ section that is linked in the delay announcement in case you are interested in seeing more information about potential refunds as well as where and how to watch the talks that were scheduled for GDC. Apparently, many of these sessions will be available online via GDC YouTube channel and the free part of GDC vault.